I just found an interview of Tohei by Aikido Journal, in which Tohei specifically states that he got the key (pun intended) from his sempai Tempu Nakamura (who was, in Tohei's words, "a yoga master and psychologist").:

I have that particular interview in my files already and it essentially credits Nakamura with some of Tohei's knowledge and slightly disparages Ueshiba's Aikido. In other places in the same interview, some of the comments about Ueshiba seem to, in my personal opinion, go a bit over the top... so I tend to mark the exactness of the assertions in that interview as being subject to question because there is evidently some amount of conflict involved.

I have no problem with the idea that Tohei learned some of the basic ki knowledge via Nakamura, though. However, since the principles of ki development are fairly immutable, I only note that this tells me that Ueshiba didn't share freely.